The Grails 3 to 4 upgrade was nowhere near as daunting as upgrading from 2.5.x. However, I thought I would do a postmortem for the upgrade just the same. This is for a project that has about 80,000 lines of Groovy/Grails, and 40,000 lines of GSP, which is quickly becoming deprecated for Angular.

Tasks:

• Custom password salt implementation finding temp workaround then replacing, with bcrypt.
• Removing a duplicate class wonder why this didn't cause issues before
• Removed a class that was just a script, which means it had a main method which caused errors
• Fixed unit test the fall back for the mixins didn't seem to work anymore
• Gorm/HQL had to replace all ? params with positional ?0,?1,..., etc.
• Had to add enabled = true to war task
• groovy.json.internal.LazyMap was being used with instanceof had to update to org.apache.groovy.json.internal.LazyMap
• Many instances where hibernate proxies were not being unwrapped. Fixed with GrailsHibernateUtil.unwrapIfProxy or fetch:'join' depending on circumstances
• DB migration plugin no longer has access to system properties so I had to add a fallback candidate to the config.
• Actuators and h2 console changed so I had to update the security around them
• Had to move where I defined the sourceSets for the resources for the db migrations
• Had to exclude old converters from grails-datastore-rest-client, which we should upgrade
• Had to fix spring security ldap:https://www.baeldung.com/spring-boot-security-autoconfigurationspring.autoconfigure.exclude=['org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration', 'org.springframework.boot.actuate.autoconfigure.security.servlet.ManagementWebSecurityAutoConfiguration']

Other resources:

• https://dev.to/erichelgeson/grails-4-upgrade-notes-3cja
• https://docs.grails.org/snapshot/guide/upgrading.html
• http://gorm.grails.org/7.0.x/hibernate/manual/index.html#upgradeNotes
• https://www.youtube.com/watch?v=i-zSKRsI2FA
• https://medium.com/agorapulse-stories/how-to-benefit-from-grails-4-upgrade-e4f4aae4a9d